1. Field of the Invention
The present invention relates to an image forming apparatus having a middle tray to temporarily store a sheet onto which an image was formed.
2. Related Background Art
In apparatus having a two-sided copy function an overlay copy function, when the two-sided or overlay copy is performed, the image on the first sheet of original is copied on a sheet and thereafter, this sheet is temporarily stored to the middle tray in the apparatus. However, the number of sheets which can be stored in the middle tray is limited and has no relation to the maximum number of continuous copies of the apparatus itself. Therefore, in execution of the two-sided copy or overlay copy function, it is impossible to continuously perform the copies above the number of sheets which can be stored to the middle tray, so that there is the drawback that the inherent performance of the apparatus cannot be effectively used.
In addition, in the case of performing two-sided copy from a plurality of originals of different sizes by such apparatus, if the image of the second original is copied onto the paper on which the image of the first original has been copied, the image of the second original may be lacking.
On the other hand, in such apparatus, there is considered the function of detecting the original size while feeding the original by the original feeding apparatus and to automatically select the cassette in which the papers of the optimum size were enclosed in accordance with the detected original size. However, in this function, it takes time until copying is started because the copy paper can be fed only after the original size was detected, so that a long time is required to copy a large quantity of originals. In particular, in the case of the originals of the same size, time is expended uselessly.
Further, when performing two-sided copy from one-side of an original, no problem will be caused if the number of originals is even. However, in the case of copying an odd number of, for example, three originals in this kind of apparatus, if they are copied from the third page, the second and third pages are formed on the front and back surfaces of a sheet of paper as shown in FIG. 7 and the first page is combined with the blank page, so that the resultant copies become unnatural.
To prevent such a situation, there is considered the method whereby the number of originals is previously counted to determine whether it is an odd number or an even number, and when it is an odd number, only the last page is copied in the one-sided copy mode and the remaining pages are copied in the two-sided copy mode. However, this method has the drawback that it takes a long time until the copy operation is started since the number of originals must be counted.
In particular, if a few originals are copied, the operator often waits for the end of copy at a location near the copying apparatus; therefore, in many cases, the operator considers the time for the counting operation as an annoying time.